core: button: allow dragging to a text-only key part

src/core/layout.rs

@@ -402,6 +402,11 @@ impl Part {
         self.key_avail = self.val.0 != Keysym::NoSymbol && key_supp;
     }
 
+    pub fn pressable(&self) -> bool
+    {
+        self.text_avail || self.key_avail
+    }
+
     pub fn modifier_id(&self) -> usize
     {
         match self.val.0 {